1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to an organic thin film transistor (OTFT) and printed metal thiol treatment.
2. Description of the Related Art
As noted in Wikipedia, printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and inkjet. Electrically functional electronic or optical inks are deposited on the substrate, creating for example, active or passive devices, such as thin film transistors or resistors. These processes can utilize any liquid phase material, including, but not limited to, solutions, mixtures, and dispersions containing organic semiconductors, inorganic semiconductors, organic dielectrics, inorganic dielectrics, metallic conductors, oxide conductors, organic conductors, nanowires, nanoparticles, nanotubes, and nanotubes.
The attraction of printing technology for the fabrication of electronics mainly results from the possibility of preparing stacks of micro-structured layers (and thereby thin-film devices) over large areas in a much simpler and cost-effective way, as compared to conventional electronics. Also, the ability to implement new or improved functionalities (e.g. mechanical flexibility) plays a role.
Organic field-effect transistors and integrated circuits can be prepared completely by means of mass-printing methods. The selection of print methods for the different layers is determined by dimensional requirements and the properties of printed materials, as well as economic and technical considerations of the final printed products. Optimal resolution of these considerations typically results in a combination of several print methods for the fabrications of the devices, as opposed to a single method.
Inkjets are flexible and versatile, and can be set up with relatively little effort. Inkjets are currently the most commonly used method for the preparation of printed electronics. However, inkjets offer a lower throughput of around 100 m2/h and lower resolution (ca. 10-20 μm) than other printing methods such as gravure. Simultaneously using many nozzles and pre-structuring the substrate permits improvements in productivity and resolution. Inkjet printing is well suited for low-viscosity, soluble materials like organic semiconductors, and has proved useful in printing dispersed particles, like inorganic metal inks, with some observed difficulties due to nozzle clogging. Because ink is deposited via droplets, thickness and dispersion homogeneity is reduced. Inkjet printing is preferable for organic semiconductors in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs) due to viscosity constraints and high ink costs, but also OFETs completely prepared by this method have been demonstrated. Frontplanes and backplanes of OLED-displays, integrated circuits, organic photovoltaic cells (OPVCs), and other devices can be prepared with inkjets.
Screen printing is appropriate for fabricating electrics and electronics on industrial scales due to its ability to produce thick layers from paste-like materials. This method can produce conducting lines from inorganic materials (e.g. for circuit boards and antennas), but also insulating and passivating layers, whereby layer thickness is more important than high resolution. Its 50 m2/h throughput and 100 μm resolution are similar to inkjets. This versatile and comparatively simple method is used mainly for conductive and dielectric layers, but also organic semiconductors, e.g. for OPVCs, and even complete OFETs can be printed.
Aerosol Jet Printing (also known as Maskless Mesoscale Materials Deposition or M3D) is another material deposition technology for printed electronics. The Aerosol Jet process begins with atomization of an ink, which can be heated up to 80° C., producing droplets on the order of one to two microns in diameter. The atomized droplets are entrained in a gas stream and delivered to the print head. Here, an annular flow of clean gas is introduced around the aerosol stream to focus the droplets into a tightly collimated beam of material. The combined gas streams exit the print head through a converging nozzle that compresses the aerosol stream to a diameter as small as 10 microns. The jet of droplets exits the print head at high velocity (˜50 meters/second) and impinges upon the substrate. Electrical interconnects, as well as passive and active components are formed by moving the print head, equipped with a mechanical stop/start shutter, relative to the substrate. The resulting patterns can have features ranging from 10 microns wide, with layer thicknesses from 10's of nanometers to >10 microns. A wide nozzle print head enables efficient patterning of millimeter size electronic features and surface coating applications. All printing occurs without the use of vacuum or pressure chambers and at room temperature. The high exit velocity of the jet enables a relatively large separation between the print head and the substrate, typically 2-5 mm. The droplets remain tightly focused over this distance, resulting in the ability to print conformal patterns over three dimensional substrates. Despite the high velocity, the printing process is gentle; substrate damage does not occur and there is generally no splatter or overspray from the droplets. Once patterning is complete, the printed ink typically requires post-treatment to attain final electrical and mechanical properties. Post-treatment is driven more by the specific ink and substrate combination than by the printing process. A wide range of materials has been successfully deposited with the Aerosol Jet process, including diluted thick film pastes, thermosetting polymers such as UV-curable epoxies, and solvent-based polymers like polyurethane and polyimide, and biologic materials.
Printed electronics allows the use of flexible substrates, which lowers production costs and allows fabrication of mechanically flexible circuits. While inkjet and screen printing are used to pattern ink onto rigid substrates like glass and silicon, mass-printing methods nearly exclusively use flexible foil, polymers and paper.
Other methods with similarities to printing, among them micro contact printing and nano-imprint lithography, are of interest. Here, μm- and nm-sized layers are prepared by methods similar to stamping with soft and hard forms. Often the actual structures are prepared subtractively, e.g. by deposition of etch masks or by lift-off processes. For example, electrodes for OFETs can be prepared in this manner. Sporadically pad printing is used in a similar manner. Occasionally so-called transfer methods, where solid layers are transferred from a carrier to the substrate, are considered printed electronics.
As mentioned above, both organic and inorganic materials are commonly used for printed electronics. These ink materials must be available in liquid form, for solution, dispersion, or suspension. Additionally, they have varying functionality, to serve as conductors, semiconductors, dielectrics, or insulators. Electronic functionality and printability can interfere with each other, mandating careful optimization. For example, a higher molecular weight in polymers enhances conductivity, but diminishes solubility. For printing, viscosity, surface tension, and solid content must be tightly controlled. Cross-layer interactions such as wetting, adhesion, and solubility as well as post-deposition drying procedures affect the outcome. Additives often used in conventional printing inks are unavailable, because they often defeat electronic functionality.
The discovery of conjugated polymers and their development into soluble materials provided the first organic ink materials. Materials from this class of polymers variously possess conducting, semiconducting, electroluminescent, photovoltaic, and other properties. Other polymers are used mostly as insulators and dielectrics. Commonly used organic conducting polymers include polymers poly(3,4-ethylene dioxitiophene), doped with poly(styrene sulfonate), (PEDOT:PSS) and poly(aniline) (PANI). Both polymers are commercially available in different formulations and have been printed using inkjet, screen, and offset printing, as well as screen, flexo, and gravure printing.
Metal inks are also commonly used in printed electronics for reasons of improved conductivity and potential for surface functionality, as compared to their organic counterparts. Silver, gold, and copper nanoparticle inks are used with all of the printing processes described above.
An organic field-effect transistor (OFET) is a transistor that uses an organic semiconductor in its channel. Organic thin film transistors (OTFTs) are the most common and scalable type of OFET. OTFTs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules. These devices have been developed to realize low-cost, large-area electronic products. Additionally, OTFTs have been fabricated with various device geometries with different combinations of bottom and top source-drain and gate contacts.
FIG. 1 is a partial cross-sectional view of a bottom gate TFT (prior art). In a bottom gate TFT a metal gate is deposited on a substrate followed by a gate insulator layer. The source and drain electrodes are patterned on the gate insulator layer. A semiconducting material is deposited onto the source and drain electrodes. In a top gate TFT the source and drain electrodes are directly deposited onto the channel (a thin layer of semiconductor), or a semiconductor material can be deposited onto the previously defined source and drain electrodes. Then, a thin film of insulator is deposited between the semiconductor and the metal gate contact. The choice of top or bottom gate structure is made based on intended application, material performance requirements, and process compatibility.
Organic polymers, such as poly(methyl-methacrylate) (PMMA), CYTOP, PVA, polystyrene, parylene, etc., can be used as a dielectric. OFETs employing numerous aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, tetracene, pentacene, diindenoperylene, perylenediimides, tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly 3-hexylthiophene (P3HT)), polyfluorene, polydiacetylene, poly 2,5-thienylene vinylene, poly p-phenylene vinylene (PPV). These can be deposited via vacuum or solution base methods, the later being of interest for printed electronics. The newer generation of solution processable organic semiconductors consists of blends of high performance small molecule and polymeric molecules for optimum performance and uniformity.
Various strategies are being devised to improve the device performance of all solution processed, printed organic transistor devices. One of the crucial elements for improving device performance is optimization of the semiconductor and source/drain electrode interfaces in order to obtain good ohmic contacts with very low contact resistance. In the case of all printed organic transistor devices, the metal is typically deposited using a solvent based ink with a number of additives that enable good printing properties of the ink. Metallic silver nanoparticle based inks are currently the most popular candidates for printing the gate and source/drain layers of the device in the case of printed organic devices. In the case of printed metals however, it is hard to get a pristine surface for two reasons: (i) the electrodes are typically thermally treated in air for 15-30 minutes to drive off the solvents and sinter the metal nanoparticles (NPs); and, (ii) the residue of solvents and additives in the inks, both of which are likely to lead to contamination on the metal surface. This leads to a non-ideal surface treatment and consequently results in poor contact properties.
The metal source drain electrodes are typically coated with a surface treatment layer (e.g., a thiol layer) in order to tune the energy level alignment and reduce the energy barrier for charge injection at the metal-semiconductor interface. These surface treatments rely on pristine metal surfaces for optimum effectiveness. A thiol treatment has proven effective in the case of pristine evaporated metal source and drain contact surfaces, but has not proven effective when applied to post-thermally treated printed metals due to the reasons mentioned above. As disclosed in the parent application entitled, ORGANIC SEMICONDUCTOR INTERFACE PREPARATION, invented by Lisa Stecker et al., Ser. No. 12/968,102, filed Dec. 14, 2010, this problem can be addressed by treating the printed metal surface with plasma, prior to applying the liquid thiol.
A thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl (—C-SH or R-SH) group (where R represents an alkane, alkene, or other carbon-containing group of atoms). Thiols are the sulfur analogue of alcohols (that is, sulfur takes the place of oxygen in the hydroxyl group of an alcohol). The —SH functional group itself is referred to as either a thiol group or a sulfhydryl group.
It would be advantageous if the surfaces of printed metals and electrodes could be treated to improve the work function and contact properties with subsequently deposited organic semiconductors.
It would be advantageous if the above-mentioned improvements could be obtained without the extra step of a plasma treatment.